A common problem encountered in the assay of haptens is that standard competitive assays produce a signal in inverse proportion to the amount of hapten in the sample suspected of containing the hapten. Thus for when testing a sample that is free of analyte a strong signal is produces whereas when testing a sample containing a significant quantity of the hapten a weak signal is produced. This can lead to difficulty in using such a test in field conditions (for example in the outdoors) and even under laboratory conditions. Also such tests need to contain a control line that can take a prolonged time to develop so that the result of the assay can be confirmed. A solution to this problem was first suggested in U.S. Pat. No. 5,641,690 where it was proposed to employ an additional antibody in the system so that a positive read out could be obtained (that is one where the grated amount of hapten in the sample the greater the signal produced).
The system described in U.S. Pat. No. 5,641,690 potentially offered a major improvement but unfortunately has not been commercialised. This has been due in part to considerable difficulties resulting from the need to prepare a specific antibody against the primary antibody in the system for each analyte envisaged. Also the backgrounds generally encountered with the embodiments of U.S. Pat. No. 5,641,690 have tended to have rendered the results less than desirable in commercial use. Furthermore no working examples of lateral flow assays (such as dip sticks) have ever been shown to work employing the system of U.S. Pat. No. 5,641,690.
Methods which result in an assay which result in the measuring of the number of binding sites not occupied by analyte are described in WO95/04231 and WO92/19973.
There is therefore a continuing need to provide an assay for a hapten in which the signal increases as the concentration of the hapten in the sample being tested increases especially one which can be produced without need to produce a new specific antibody for each analyte as part of the signal generating system and preferably which can be formatted as a lateral flow assay such as a dip stick. Such an assay has now been found as have devices for performing the assay and components to use in the assay.